1. Field of the Invention
The invention relates to a method for stabilizing low dielectric constant materials, and more particularly to a method for stabilizing spin on polymer materials.
2. Description of the Prior Art
It is the nature of semiconductor physics that as the feature sizes are scaled down, the performance of internal devices in integrated circuits improves in a compounded fashion. That is, the device speed as well as the functional capability improves. The overall circuit speed, however, becomes more dependent upon the propagation speed of the signals along the interconnects that connect the various devices together. With the advent of very and ultra large scale integration (VLSI and ULSI) circuits, it has therefore become even more important that the metal conductors that form the interconnections between devices as well as between circuits in a semiconductor have low resistivities for high signal propagation. Copper is often preferred for its low resistivity, as well as for resistance to electromigration and stress voiding properties.
On the other hand, considerable attention has focused on the replacement of silicon dioxide with new materials, particular material having lower dielectric constants, since both capacitive delays and power consumption depend on the dielectric constant of the insulator. Accordingly, circuit performance enhancement has been sought by combining the copper conductors with low dielectric constant layer (k less than approximately 4).
Spin on polymer, for example, aromatic hydrocarbon material contained or SiLK, has been used recently as a low dielectric constant material. Those spin on polymers mostly have hydrophobic behavior after a curing step. For the hydrophobic nature of the polymers, they have the tendency to chemically protect moisture or solvents on their surface. Even so, the physical absorption on a film""s surface is still observed from environments. The absorption phenomena would result in degradation of the characteristics of the polymers, such as an increase of the dielectric constant. Depicted in FIG. 1, the value of the dielectric constant changes with respect to the exposure time of the polymers. The result clearly indicates that the dielectric constant gradually increases with the exposure time after the curing step. On the other hand, the moisture or solvents may be removed by using a baking or degassing step. However, the polymers have loose structures that may result in re-adsorption phenomena when the polymers are again exposed for a long time.
One of the objectives of the present invention is to provide a method to stabilize low dielectric constant materials. Formation of a passivated film on the low dielectric constant materials can protect them from adsorption of moisture or solvents after the curing step.
Another one of the objectives of the present invention is to provide a method to stabilize spin on polymer materials. The surfaces of the spin on polymer materials are treated with alkaline solution.
In the present invention, a method for stabilizing low dielectric constant materials in a semiconductor structure is disclosed. The method comprises providing the semiconductor structure and thereon spinning-on a dielectric layer. After a curing step, the dielectric layer is treated with an aqueous solution containing ammonium hydroxide. With the aqueous solution, a passivated film formed from the surface of the dielectric layer, such as a polymer layer, can protect the dielectric layer from adsorption of moisture or solvents.